The single greatest source of energy loss in a building is generally considered to be through the window areas. This is caused in large part by conductive heat transfer through glass window panes, i.e., heat transmitted from one side of the glass pane to the other. Air movement through gaps that may exist between the window elements, such as between the window pane and sash, also contribute to heat transfer through the window area. Energy loss through the window areas from such a temperature and/or pressure gradient existing between the inside and outside of the windows is particularly aggravated in older buildings, such as older homes, having single-pane windows which have become loosely fitted in their sashes with age and use.
It is known that heat conduction can be greatly inhibited through the use of two thicknesses of glass with an air space or gap formed between the two glass panes. This air gap forms an insulative barrier between the two glass panes.
Efforts have been made to insulate existing windows by adding an additional pane of glass to that already in place in the original window structure. Some of these attempts have taken the form of factory-manufactured insulated window units, which can only be installed in certain kinds of window frames, typically require professional installation, and are generally not well suited for cost efficient residential use, in particular. Exterior aluminum storm windows are available, but also require substantial installation effort, are considered to be unattractive, may provide little or no insulating value, and constitute only a minor barrier to air infiltration.
Rigid window units of the foregoing types are, in general, also not readily adjustable to accommodate varying window frame dimensions, even in "standard" window frame sizes.
Non-glass window insulation kits are available which typically constitute a sheet of thin plastic film that can be fastened by tape, magnets, etc. around the outside of a window frame. These latter plastic film thermal "windows" are generally unsightly, require substantial effort to install properly, and typically do not last long.